[PROPOSAL] Replace secp256k1 with pure Rust package libsecp256k1

[russellwmy]

Problem

The core near crypto package uses the C wrapper librarysecp256k1. However, this library does not able to support compile in wasm32. This issue may not fix in short term as the developer state the issue here.

Proposal

Instead, I propose replacing the library with libsecp256k1. libsecp256k1 is developed by Parity Technologies. It is pure Rust implementation. It supports compile in wasm32.